I recently volunteered to help with a local parade. My main responsibility was to help direct traffic: basically, I stopped each vehicle, and asked them if they were participating in the parade. If they said no, I told them they had to turn around because the road was closed. One person ignored me after I told them to turn around and kept going. Several people didn't stop when I motioned for them to stop, so I didn't get a chance to tell them to turn around. Did they break any laws?
Pursuant to ORS 811.445,
A person commits the offense of use of a throughway when prohibited if any use restrictions or prohibitions are posted by appropriate signs giving notice thereof and the person violates any restriction or prohibition so posted
and you get a traffic ticket for violation. A narrow interpretation of the word "sign" is some physical object with appropriate words like "Road closed for parade", but a broader interpretation is "an indication", such as a person saying "Road closed for parade". Nothing in state law restricts this notification power to only uniformed officers. However, ORS 811.535 which is the general "failure to obey" law only applies to disobeying uniformed police officers displaying a badge. The difference between the two offenses is that violating "use when prohibited" is only a class D infraction while disobeying a uniformed police officer is a class B infraction.
State law does not say what kind of person can direct traffic, it frames restrictions in terms of what a traffic authority can regulate (such as parades). County or municipal ordinances might then specifically say who is authorized to direct traffic. I assume it was not unknowable that there was a parade then and there, so the driver was already on notice that the road was closed to regular traffic. Nevertheless, state law does require that there be a "sign", so if you had called the police to ticket a person, they might have made a federal or at least state case out of it, and we could learn what, in Oregon, the word "sign" means in the context of that law.
A crucial question is whether the road was closed by a competent traffic authority. That would be the county, I presume, and not e.g. the local Amvets chapter.